1. Field of the Invention
The present invention relates to a dark field illumination device of a microscope.
2. Description of the Related Art
Dark field observation is generally used when a sample containing pure substances having translucency and optical anisotropic substances is observed with a microscope.
The dark field observation is a method of observing only scattered light from the sample by preventing illumination light from directly entering observation optical system, so that, in an observed image, only the scattered light can be seen in the darkness.
At this point, if light rays other than the scattered light of the sample enters the observation optical system, a background of the observed image will not be completely dark, leading to decreased contrast and making it difficult to detect the scattered light of the targeted sample. It is thus important to prevent unnecessary light rays from entering the observation optical system in the dark field observation.
Jpn. UM. Appln. KOKOKU Publication No. 45-11051 discloses an illumination device for performing the dark field observation. FIG. 5 shows a sectional configuration of the illumination device for performing the dark field observation disclosed in this document.
In the illumination device shown in FIG. 5, a switch lever 22 for bright field/dark field is placed above an illumination light source 18 during dark field observation. This shields only the illumination light rays directly entering the observation optical system among the illumination light rays emitted from the illumination light source 18. Other illumination light rays are reflected toward a sample 30 by an inner surface of a reflecting mirror 19, and illuminate the sample 30. At this moment, in order to prevent light rays in light paths other than designed light paths from striking on the switch lever 22 for bright field/dark field and being scattered to enter the observation optical system, a light shielding portion 31 is disposed to prevent the light rays in light paths other than the originally designed light paths from entering the observation optical system.
The unnecessary reflected illumination light rays that have penetrated the sample 30 moves upward in the drawing without entering the observation optical system. As a result, the dark field observation can be achieved in which only the scattered light of the observed image can be seen in the dark field.
Furthermore, Jpn. Pat. Appln. KOKAI Publication No. 11-153755 discloses an illumination device for performing the dark field observation. FIG. 6 shows a sectional configuration of the illumination device for performing the dark field observation disclosed in this document.
In the illumination device shown in FIG. 6, a light shielding portion 36 is disposed between an illumination light source 34 and an objective lens 39 as the observation optical system. The light shielding portion 36 only shields illumination the light rays directly entering the objective lens 39 among the illumination light rays emitted from the illumination light source 34. Other illumination light rays are reflected toward a sample 38 by an inner wall surface 35a formed inside a reflecting mirror 35, and illuminate the sample 38.
The disadvantageous reflected illumination light rays that have penetrated the sample 38 moves upward in the drawing without entering the objective lens 39. As a result, the dark field observation is achieved in which only the scattered light of the observed image can be seen in the darkness.
In many cases, the sample is held with a glass plate or a glass container. Moreover, a dustproof waterproof glass to prevent entrance of dust and water into the illumination device is provided between the sample and a light shielding plate. Thus, a glass member is generally present between the sample and the illumination light source. Therefore, part of the illumination light rays that illuminate the sample is again reflected toward the illumination light source by the glass surface. Part of the reflected light rays falls on the light shielding plate and is diffused, and part of the diffused light enters the objective lens to decrease the contrast of the observed image.
Furthermore, as the scattered light emitted from the sample is faint in the dark field observation, improvements are increasingly made in brightness of an illumination system, such as employment of a brighter light source or adjustment of the illumination system. This, on the other hand, increases an amount of light rays that are again reflected toward the light source side by the glass surface, so that the decrease of contrast mentioned above has become a more significant problem.